The present invention relates to a device for forming castings such as a bearing seat for a bearing, a blower boss or a wheel boss, wherein a helical coil is embedded in the casting so that the thermal expansion coefficient of the bearing supporting portion of the casting is defined by the coil. The invention relates further to a manufacturing method for such castings and a coil per se to be used in combination with the device and the method. I
Devices and methods of this kind are described in, for example, Swedish Patent 7405905-6 and U.S. Pat. No. 4,065,190 (corresponding to Swedish Patent 8202045-4) the disclosures of which are hereby incorporated by reference into the present description. The method of manufacturing the spring into a complete finished product will be briefly described. A steel wire having an untreated or treated surface, is folded and rolled to form a helical coil. The coil is then covered by a suitable plastic material which forms a bearing seat. During the warming-up of the coil surface by the plastic material, the stresses previously built into the coil by the folding and rolling steps begin to be released. This stress release starts at about 70.degree. C. A normal material temperature of the plastic material is about 260.degree. C. The stress release causes the coil to be gradually deformed, whereby its geometric dimensions become changed from those required of the coil in performing its intended function in the finished product.
In the injection casting mold the coil is kept in position by a coil retainer which is withdrawn shortly before the injection is terminated to create a cavity which is then filled with plastic, the coil being embedded within the plastic casting. The plastic material injected into the mold creates in the mold a pressure in the magnitude of 500 kg/cm.sup.2 acting on the coil. Furthermore, the incoming plastic material speed is very high. The high pressure and high speed may cause the coil position to be changed, and also produce a somewhat uncontrollable movement of both coil ends. Due to the dimensional changes of the coil, and the pressure effect mentioned above, the coil can be moved in such a way that it is not situated in the desired position in the casting.
Prior to being placed in the mold, the coil surface is subjected to a conventional phosphatizing treatment to prevent corrosion. Also, prior to being formed into a coil, the wire is provided with some sort of lubrication on the surface thereof to facilitate the rolling-up into a coil. Therefore, it will be appreciated that even if the original wire had a clean surface, it is soiled after being manufactured into a coil. The greasy or unclean surface inhibits cast-in material from adhering to the coil.
A purpose of the present invention is to ensure that the cast-in coil retains its intended shape and position after injection.
Another purpose of the invention is to ascertain that a good adhesion is obtained between the coil and the castings.